Active damping for synchronous generator torsional oscillations

ABSTRACT

A generator control unit (GCU) provides active damping of a synchronous generator by monitoring the speed of the synchronous generator and detecting oscillations in the monitored speed. The oscillations are indicative of torsional oscillations within the mechanical drivetrain including the synchronous generator or generators. In response to detected oscillations in the monitored speed, the GCU generates a varying set-point value that is used to control the excitation voltage provided to the synchronous generator. Varying the excitation voltage provided to the synchronous generator causes a variation in synchronous generator torque. By selectively varying the torque in the synchronous generator, the GCU provides active damping in the synchronous generator that decreases or dampens the torsional oscillations.

BACKGROUND

The present invention relates to synchronous generators, and inparticular to active damping of synchronous generators.

Synchronous generators are used in a variety of applications to convertmechanical energy provided by an engine to alternating current (AC)electrical energy. In particular, synchronous generators are used inapplications such as aboard aircraft to generate the AC electricalenergy necessary to support on-board electrical systems.

In a typical topology, an engine generates mechanical energy that isprovided through a gearbox to a synchronous generator or to multiplesynchronous generators. A shaft transmits the mechanical energy from thegearbox to the synchronous generator. Due to a multitude of competingmechanical design considerations, the shaft may be relatively long andmechanically compliant. The inertias associated with the engine, thegearbox, the synchronous generator, and other gearbox driven accessoriesin combination with the mechanical compliance or spring rates of themechanical drivetrain, including the generator shaft, create adistributed mechanical spring-mass system that has associated torsionalresonances. There are multiple torsional modes and associated resonancesthat involve the generators for multiple direct-driven generators on acommon gearbox. Engine gearboxes typically exhibit very lightly dampedcharacteristics, and because the synchronous generator is controlled tomaintain an AC voltage, it presents a near constant power loadcharacteristic to the mechanical drivetrain that results in negativedamping for disturbance frequencies that are within the generator'svoltage regulation bandwidth. In certain situations, depending on thegenerator speed, the generator electrical load, and the net effectivedamping in the overall mechanical drivetrain, the torsional resonance ofthe spring-mass system involving the generator or generators can lead tolarge, undesirable torsional oscillations and mechanical failures in thesystem. Mechanical damping may be used to offset the negative dampingcharacteristic of the synchronous generator or generators and thusdampen the torsional oscillations in the spring-mass system, butmechanical damping requires additional parts that increase the weightand cost of the system.

SUMMARY

A controller for a generator provides active damping by monitoring thespeed of the generator and detecting oscillations in the monitored speedthat indicate the presence of torsional oscillations. In response todetected oscillations in the monitored speed, the controller dynamicallyvaries a set-point associated with the generator. Varying the set-pointresults in variation of the torque in the synchronous generator thatdampens or decreases the torsional oscillations within the generator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a system that includes anengine/gearbox connected to provide power to a synchronous generator anda generator control unit connected to control the output voltage of thegenerator as well as provide active damping to the system.

FIG. 2 is a block diagram illustrating in more detail the connection ofthe synchronous generator to the generator control unit.

FIG. 3 is a block diagram illustrating functions performed by thegenerator control unit to provide active damping to the system.

DETAILED DESCRIPTION

A synchronous generator is controlled by a generator control unit inorder to maintain a constant value output voltage despite changingelectrical loads and gradual increases and decreases in the speed of thegenerator associated with normal engine operation. In general, thegenerator control unit regulates the output voltage of the synchronousgenerator by controlling an excitation voltage provided to thesynchronous generator. In addition, the generator control unit controlsthe excitation voltage provided to the synchronous generator to provideactive damping to the mechanical system including the synchronousgenerator. The generator control unit provides active damping bymonitoring the speed associated with the synchronous generator anddetecting oscillations within the monitored speed that are indicative oftorsional oscillations. In response to the detected oscillations, thegenerator control unit dynamically modifies the excitation voltageprovided to the synchronous generator such that the torque associatedwith the generator is selectively varied. By selectively varying thetorque in proper phase relationship to the generator speed oscillationsthe generator provides active or positive damping that dampens orreduces the torsional oscillations in the generator. A benefit of usingelectrical means to provide damping (as opposed to mechanical means) isno additional hardware or mechanical components are required.

FIG. 1 illustrates a system for converting mechanical energy toelectrical energy. Components included within the system includeengine/gearbox 10, shaft 12, synchronous generator 14, and generatorcontrol unit (GCU) 16. Engine/gearbox 10 generates mechanical energyusing any number of well-known methods. In addition, although it iscommon in direct-driven systems to include a gearbox to scale the speedprovided by engine 10, mechanical energy may be provided to synchronousgenerator 14 in a variety of ways.

Shaft 12 transmits mechanical energy provided by engine/gearbox 10 tosynchronous generator 14. Based on the mechanical energy provided byengine/gearbox 10, synchronous generator 14 generates an electricaloutput that is provided to one or more electrical loads. To maintain adesired output voltage, GCU 16 monitors the output voltage ofsynchronous generator 14 and regulates an excitation voltage provided tosynchronous generator 14. In an exemplary embodiment, GCU 16 isprogrammed with a voltage set-point that defines the desired output rootmean square (rms) voltage of synchronous generator 14. In this way, GCU16 maintains the rms value of the output voltage of synchronousgenerator 14 at a desired value as determined by the voltage set-point.

The system including engine/gearbox 10, shaft 12 and synchronousgenerator 14 form a spring-mass system that has the potential togenerate torsional oscillations. In addition to controlling the outputof synchronous generator 14 by controlling the magnitude of theexcitation voltage provided to synchronous generator 14, GCU 16 alsoprovides active or positive damping of synchronous generator 14. Asdiscussed in more detail with respect to FIGS. 2 and 3, GCU 16 providespositive damping by varying the voltage set-point such that the torqueof synchronous generator 14 is selectively varied at the same frequencyas the detected torsional oscillations, creating a varying torque thatacts to dampen torsional oscillations in synchronous generator 14.

FIG. 2 shows an embodiment of connections between synchronous generator14 and GCU 16. In this embodiment, synchronous generator 14 includesmain synchronous generator 20, rotating rectifier assembly 22, exciter24, permanent magnet generator (PMG) 26, and current transformers 28.Main synchronous generator 20, exciter 24, and PMG 26 are each connectedto receive mechanical energy from shaft 12. In response to mechanicalenergy provided by shaft 12, PMG 26 generates a 3-phase alternatingcurrent (AC) output voltage that is provided to GCU 16. Circuitryincluded within GCU 16 rectifies the 3-phase AC voltage to a directcurrent (DC) value and then regulates the DC value to a desiredmagnitude that is provided as an excitation voltage to exciter 24. GCU16 regulates the magnitude of the excitation voltage based on monitoringthe output 3-phase AC voltage generated by main synchronous generator 20(as measured at line contactor 32) and the voltage set-point. Inresponse to mechanical energy provided by shaft 12 and the magnitude ofthe excitation voltage provided by GCU 16, exciter 24 generates a3-phase AC output voltage that is provided to rotating rectifierassembly 22, which rectifies the 3-phase AC voltage and provides theresulting DC voltage as an excitation voltage to main synchronousgenerator 20. In response to the mechanical energy provided by shaft 12,and the excitation voltage provided by exciter 24, main synchronousgenerator 20 generates a 3-phase AC output voltage that is providedthrough generator current transformer 28 and line current transformers30 to line contactor 32. GCU 16 therefore monitors the 3-phase AC outputvoltage generated at line contactor 32 (as well as the line current insome instances) and controls the DC excitation voltage provided toexciter 24 such that the rms value of the 3-phase AC output voltage atline contactor 32 is maintained at a desired value (i.e., the voltageset-point value).

To provide active damping to synchronous generator 14, GCU 16 must firstdetect the presence of torsional oscillations. By measuring the speed ofsynchronous generator 14 and detecting oscillations within the measuredspeed the presence of torsional oscillations may be detected. In anexemplary embodiment, oscillations are detailed within a frequency rangethat is characteristic of the mechanical torsional resonances that arepresent in the particular mechanical drivetrain. In the embodiment shownin FIG. 2, GCU 16 is able to detect the presence of torsionaloscillations by indirectly measuring the speed of synchronous generator14 based on the AC output voltage provided by PMG 26. That is, thefrequency of the AC voltage provided by PMG 26 is directly related tothe speed at which PMG 12 rotates.

In other embodiments, rather than indirectly measure the speedassociated with synchronous generator 14 based on the AC output voltage,other methods of measuring the speed associated with synchronousgenerator 14 may be employed. In an exemplary embodiment, an observerstructure that includes a model of synchronous generator 14 may beemployed. For example, in one embodiment the observer structure isimplemented with a Luenberger observer that generates a speed estimatefor synchronous generator 14 based on sensed terminal voltage currentsand voltages. In general, the Luenberger observer monitors changes inthe voltage in response to changes in generator speed, and includes amodel for generating a predicted or estimated voltage. The monitoredvoltage and the estimated voltage are compared to generate an estimateof actual generator speed. In addition, the Luenberger observer makesuse of current measurements to account for changes in the output voltagecaused by variations in the load (as opposed to variations caused bytorsional oscillations). In this way, the Luenberger observer providesan alternative method of sensing generator speed. In another embodiment,the observer structure may be implemented with a Kalman filter thatestimates the speed of the generator. In still other embodiments, aspeed sensor or equivalent device may be used to directly observe ormeasure the speed associated with synchronous generator 14.

In the embodiment shown in FIG. 2, detected oscillations within themeasured speed are indicative of torsional oscillations. In particular,torsional oscillations cause the speed of synchronous generator 14 tovary in an approximate sine wave fashion about the average speed at acertain frequency (e.g., 40 Hertz (Hz)). For example, if the averagespeed of synchronous generator 14 is 10,000 revolutions per minute(RPM), then torsional oscillations may result in the shaft speedincreasing to 10,100 RPM and decreasing to 9,900 RPM at a frequency of40 Hz. In response to detected torsional resonant frequency oscillationsin the speed of synchronous generator 14, GCU 16 responds by varying thevoltage set-point (e.g., the value representing the desired rms value ofthe AC output voltage of synchronous generator 14) to actively dampenthe torsional oscillations. Specifically, GCU 16 increases and decreasesthe voltage set-point at the frequency of the torsional oscillations. Inthe above example, GCU 16 varies the set-point at a frequency of 40 Hz,and increases the set-point above the nominal value when the torsionaloscillations cause the speed of synchronous generator 14 to increaseabove the average speed, and decreases the set-point below the nominalvalue when the torsional oscillations cause the speed of synchronousgenerator 14 to decrease below the average speed. Increasing the voltageset-point causes the mechanical torque of synchronous generator 14 toincrease. Likewise, decreasing the voltage set-point causes themechanical torque of synchronous generator 14 to decrease. Varying thetorque of synchronous generator 14 at the same frequency and in correctphase with the torsional oscillations results in synchronous generator14 providing active or positive damping that reduces or dampens thetorsional oscillations in the mechanical drivetrain.

FIG. 3 illustrates an exemplary embodiment of the functional operationsperformed by GCU 16 to provide active damping of synchronous generator14. In this embodiment, GCU 16 is implemented by a microprocessor ordigital signal processor (DSP) that performs the functions illustratedin FIG. 3 using a combination of software and hardware components. Inother embodiments, the functions illustrated in FIG. 3 are implementedusing analog devices. In addition, functional operations shown in FIG. 3are limited to those functions related to providing active damping tosynchronous generator 14. In other embodiments, GCU 16 may includeadditional functionality beyond those functions illustrated in FIG. 3.

As shown in FIG. 3, GCU 16 includes rectifier 34, DC-DC converter 36,speed estimator 38, high-pass filter 40, signal compensation 42, summer44, and voltage regulator 46. Rectifier 34 rectifies the AC outputvoltage generated by PMG 26 (as shown in FIG. 2) to a DC voltage that isprovided to DC-DC converter 36. In the situation in which no torsionaloscillations are detected, voltage regulator 46 acts to control themagnitude of the DC output voltage generated by DC-DC converter 36 basedon the voltage set-point value and the monitored output voltagegenerated by main synchronous generator 20 (as measured at linecontactor 32 shown in FIG. 2). By selectively increasing or decreasingthe output voltage generated by DC-DC converter 36 the output voltagegenerated by main generator 20 can be selectively controlled to adesired level. The operations performed by GCU 16 illustrate a simplemodel used to maintain the output voltage of main synchronous generator20 at a desired level. In other embodiments, more complex algorithms andadditional inputs may be used to control the output voltage of mainsynchronous generator 20.

In addition, the AC output voltage generated by PMG 26 is provided tospeed estimator 38. Because the signal provided by PMG 26 is analternating current signal with a frequency that is directly related tothe speed of PMG 26, speed estimator 38 is able to determine the actualspeed of PMG 26, and thus the speed of synchronous generator 14, basedon the zero-crossings (i.e., the frequency with which the AC outputvoltage crosses the value zero). For instance, in an exemplaryembodiment, speed estimator 38 includes a high-frequency clock thatmeasures the period between successive zero crossings of the AC outputvoltage of PMG 26. Measuring the period between successive zerocrossings allows speed estimator 38 to estimate the actual speed ofsynchronous generator 14. In addition, to ensure accuracy in estimatingthe speed of synchronous generator 14, the speed estimator may averagesuccessive cycles together in order to account for electromechanicalasymmetries in the design of PMG 26. For instance, PMG 26 may include anumber of permanent magnets that are spaced very nearly, but not exactlyequal, relative to one another. Averaging the estimated speed overseveral successive cycles provides a more accurate estimate of theactual speed of synchronous generator 14. In other embodiments an actualspeed sensor may be employed to monitor the speed of synchronousgenerator 14 and provide an input to GCU 16 reflecting the measuredspeed.

The monitored speed of synchronous generator 14 is provided to high-passfilter 40, which detects oscillations associated with the speed ofsynchronous generator 14. As discussed above, the presence ofoscillations in the speed of synchronous generator 14 is indicative oftorsional oscillations. For example, if torsional oscillations exist onsynchronous generator 14, such that the speed of synchronous generator14 oscillates between 9,900 RPM and 10,100 RPM at a frequency of 40 Hz,then high pass filter 40 isolates the 40 HZ oscillation within themeasured speed. In this way, DC variations in the speed of synchronousgenerator 14 (such as those caused by variations in the mechanicalenergy generated by engine 10) are filtered out, with only theoscillations indicative of torsional oscillations being provided tosignal compensator 42.

The torsional oscillations detected within synchronous generator 14 areprovided to signal compensator 42, which provides dynamic compensationand scaling of the oscillations. In particular, signal compensator 42modifies the phase of the torsional oscillations to counteract delays inmeasuring and processing the speed oscillation signal. For instance, inthe embodiment described above in which speed estimator 38 averagesspeed estimates provided over several cycles of data, the delay causedin estimating the speed of synchronous generator 14 is compensated forby signal compensator 42. In this way, signal compensator 42 ensuresthat the signal ultimately provided to voltage regulator 46 is properlyphased such that GCU 16 provides active damping (i.e., the variations ingenerator torque are properly phased to dampen torsional oscillations).

Signal compensator 42 also provides scaling to the torsionaloscillations provided by high-pass filter 40. The scaling provided bysignal compensator 42 ensures that the magnitude of the signal added tothe voltage set point by adder 44 is properly scaled to provide adequateactive damping. Generally speaking, the magnitude of the torsionaloscillation signal added to the voltage set-point must be keptcomparatively small (relative to the voltage set-point), such thatadding the torsional resonant frequency oscillation signal to thevoltage set-point does not cause the output voltage of main synchronousgenerator 20 to increase or decrease beyond defined tolerances. In thisway, the output of main synchronous generator 20 is still maintained ata relatively constant value (as required by electric power qualityspecifications) but torsional oscillations within synchronous generator14 and shaft 12 are damped or reduced.

The resulting signal generated as a result of adding the scaled,compensated torsional oscillation signal to the voltage set-pointresults in a varying voltage set-point value being provided to voltageregulator 46. In addition, monitored line voltage representing theoutput voltage of main generator 20 is also provided to voltageregulator 46. Based on these inputs, voltage regulator 46 generates aduty cycle signal that is provided to DC-DC converter 36. In particular,the variation of the voltage set-point caused by the addition of thetorsional oscillation signal results in voltage regulator 46 introducingan oscillating component to the excitation voltage generated by DC-DCconverter 36. The ramping voltage set-point results in a generatortorque varying at the same frequency as the detected torsionaloscillations. The varying generator torque, if maintained at the properphase relative to the speed oscillations of synchronous generator 14,dampens or decreases the torsional oscillations in synchronous generator14.

Thus, GCU 16 monitors the speed of synchronous generator 14, andisolates within the monitored speed oscillations indicative of torsionalresonance. The torsional oscillation component of the measured speed isscaled and phase compensated such that the resulting value can be addedto the voltage set-point to generate active damping within generator 12.Specifically, as the monitored speed increases (i.e., the torsionaloscillation component of the monitored speed increases as part of theapproximate sine wave oscillation) the resulting set-point valueprovided to voltage regulator 46 is increased such that increased torqueis generated within synchronous generator 14. Likewise, as the monitoredspeed decreases (i.e., the torsional oscillation component of themonitored speed decreases as part of the approximate sine waveoscillation) the resulting set-point value provided to voltage regulator46 is decreased such that synchronous generator 14 torque is decreased.By varying torque associated with generator 14 at the same frequency asthe detected oscillations, synchronous generator 14 provides activedamping that decreases the torsional oscillations in the mechanicaldrivetrain.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A synchronous generator system comprising: a synchronous generatoroperatively connectable to receive mechanical input from a shaft and toprovide electrical output in response to the mechanical input; and agenerator control unit (GCU) operatively connected to monitor the outputvoltage generated by the synchronous generator and to selectivelycontrol an excitation voltage provided to the synchronous generator tomaintain the output voltage at a desired magnitude, wherein the GCUprovides active damping within the synchronous generator by monitoring aspeed of the synchronous generator and in response to oscillationsdetected within the monitored speed, generating a varying excitationvoltage that is provided to the synchronous generator.
 2. Thesynchronous generator system of claim 1, wherein the GCU includes: aspeed sensing circuit that monitors an alternating current (AC) voltageassociated with the generator and estimates speed of the synchronousgenerator based on the monitored AC voltage.
 3. The synchronousgenerator system of claim 2, wherein the speed sensing circuit monitorszero crossings of the AC voltage to determine the speed associated withthe synchronous generator.
 4. The synchronous generator system of claim1, wherein the speed sensing circuit includes an observer structure thatdetermines the speed associated with the synchronous generator based onthe monitored AC voltage, a monitored AC current, and a generator modelthat generates a predicted voltage estimate.
 5. The synchronousgenerator system of claim 1, wherein the GCU includes: a filter circuitthat isolates the oscillations in the monitored speed of the synchronousgenerator.
 6. The synchronous generator system of claim 5, wherein theGCU includes: a signal compensation circuit that modifies the phase andscale of the oscillations isolated by the filter circuit.
 7. Thesynchronous generator system of claim 1, wherein the GCU includes: asummer circuit that generates a varying value set-point based on aconstant value set-point and the oscillations detected within themonitored speed; and a voltage regulator circuit that varies theexcitation voltage provided to the generator based on the varying valueset-point provided by the summer circuit.
 8. A method of providingactive damping to a generator, the method comprising: monitoring speedassociated with the synchronous generator; detecting oscillations withinthe monitored speed; and varying an excitation voltage provided to thesynchronous generator based, in part, on the detected oscillations inthe speed of the synchronous generator, wherein the excitation voltageis varied at a frequency that is based on the frequency of the detectedoscillations.
 9. The method of claim 8, wherein monitoring the speed ofthe synchronous generator includes: receiving input from a speed sensorconnected to monitor speed associated with the synchronous generator.10. The method of claim 8, wherein monitoring the speed of thesynchronous generator includes: monitoring an alternating current (AC)voltage generated by the synchronous generator; detecting a frequencyassociated with the monitored AC voltage; and estimating the speed ofthe synchronous generator based on the frequency of the monitored ACvoltage.
 11. The method of claim 8, wherein monitoring the speed of thesynchronous generator includes: monitoring a terminal voltage associatedwith the synchronous generator; monitoring a terminal current associatedwith the synchronous generator; generating an estimated terminal voltagebased on a model of the synchronous generator; and determining the speedassociated with the synchronous generator based on the terminal voltage,the terminal current and the estimated terminal voltage.
 12. The methodof claim 8, wherein detecting oscillations within the monitored speedincludes: providing the monitored speed to a high-pass filter thatisolates the torsional oscillations within the monitored speed.
 13. Themethod of claim 8, wherein varying an excitation voltage provided to thesynchronous generator based, in part, on the detected torsionaloscillations in the speed of the synchronous generator includes:providing phase compensation to the isolated torsional oscillations tocompensate for time delays in monitoring the speed associated with thesynchronous generator; adding the phase compensated torsionaloscillation signal to a voltage set-point to generate a varying voltageset-point value; and varying the excitation voltage provided to thesynchronous generator based on the varying voltage set-point value. 14.A controller for providing active damping to a system that includes asynchronous generator, the controller comprising: a speed sensingcircuit that measures a speed of the synchronous generator; a detectioncircuit that detects torsional oscillations in the measured speed of thesynchronous generator; and a voltage regulator circuit that varies anexcitation voltage provided to the synchronous generator based, in part,on the detected torsional oscillations in the speed of the synchronousgenerator, wherein the voltage regulator circuit varies the excitationvoltage at a frequency equal to the frequency of the detected torsionaloscillations.
 15. The controller of claim 14, wherein the speed sensingcircuit is connectable to receive speed measurement data from a speedsensor connected to monitor speed of the synchronous generator.
 16. Thecontroller of claim 14, wherein the speed sensing circuit is connectableto monitor an alternating current (AC) voltage generated by thesynchronous generator, wherein the speed sensing circuit indirectlymeasures the speed of the synchronous generator based on a measuredfrequency of the monitored AC voltage.
 17. The controller of claim 14,wherein the speed sensing circuit is connectable to monitor an ACvoltage and an AC current generated by the synchronous generator,wherein the speed sensing circuit includes a Luenberger observer thatmeasures the speed of the synchronous generator based on the monitoredAC voltage and AC current.
 18. The controller of claim 14, wherein thedetection circuit includes: a high-pass filter circuit that isolates thetorsional oscillations in the measured speed of the synchronousgenerator.
 19. The controller of claim 18, further including: a signalcompensation circuit that adds phase compensation to the torsionaloscillations to ensure the varying voltage set-point is properly phasedto provide active damping.
 20. The controller of claim 19, furtherincluding: a summing circuit that generates a varying voltage set-pointvalue based on the torsional oscillations provided by the signalcompensation circuit and a constant voltage set-point that represents adesired output voltage of the synchronous generator; and a voltageregulator that controls the excitation voltage provided to thesynchronous generator based on the varying voltage set-point value.